Composition for lithographic plate coatings



Patented May 4, 1954 UNITED STATES PATENT OFFICE COMPOSITION FOR LITHOGRAPHIO PLATE COATINGS N Drawing. Application November 10,1950, Serial No. 195,132

4 Claims. 1

This invention relates to image forming coatings for lithographic plates, and constitutes an improvement over the coating .disclosed in the copending application of Vernon ,L. Gregory, Serial No. 195,131, filedNovember 10, 1950.

In the aforesaid application, an image forming coating for photolithographic plates was disclosed in which a preferably natural fatty emulsion was included in the light sensitive colloid composition. Due to this added fat content in the coating ink receptivity thereof was increased, and additional copies or reproductions could thereby be obtained. However, it was found that while the coating compositions set forth in said application were superior to those previously used in the art, they also possessed certain features of disadvantage. One of suchshortcomings was the difficulty of obtaining uniformly good results even when the same ratio of a fatty emulsion was used; resulting in part from absolutely undesirable free fatty acid rendering the unexposed portions of the lithographic plate, as well as the exposed portions, receptive to ink, thereby resulting in indistinct or slightly blurred reproductions. Furthermore, after conducting numerous tests, it was ascertained that even after exposure the coating, due to its fat content, remained too soft to produce the highest qualityprints. However, if the fat content were reduced sufficiently to overcome this difficulty, the desirable ink receptivity, resulting from the use of fat, was substantially reduced. 'It should also be pointed out that, as was mentioned in said copending application, it is preferable to prepare the coating composition in dried powder form, and merely require the addition of water to obtain the proper coating solution. However, with the colloids heretofore such as casein, the dried mixture would not readily dissolve in water Without the addition of some hydroxyl compound, thereby increasing and making more vdifiicult the task of the user of the product. From the foregoing it will be seen that certain problems have been encountered in previousiy used compositions, and the aforesaid disclosure of the addition of a fatty emulsion to such compositions has raised or presented new diihculties heretofore not found in the art.

An object or feature of the present invention is the overcoming of the previously discussed difficulties and disadvantages and the provision of an image forming coating for lithographic plates which is sufficiently hard to produce distinct reproductions in spite of the substantial amount of a fat contained therein.

Another object of our invention is to provide a coating of the type described which may be prepared in dried form and which may readily be dissolved in water without .the addition of any other compounds not essential to the coating composition.

A further object of this invention is to provide an improved colloid for use in a lithographic plate coating containing a fatty emulsion which will effectively and simply prevent a-soft image, which might otherwise be caused by the fat in the coating.

The invention possesses other objects and features of advantage which, together with the foregoing, will be specifically set forth in the following detailed description of the invention. It will be understood that our invention is not to be limited to the specific composition or procedure described herein, as variations thereof may be employed which come within the scope of the appended claims.

In broad terms, the present invention relates to an improved image forming coating for photolithographic plates, and includes the use of a water-soluble .caseinate in such a composition. As is well known in the art, the typical composition. for such coatings comprises a colloid rendered light sensitive by the addition of a light sensitive salt such as ammonium bichromate, or the sodium or potassium salts of bichromate. The colloids most frequently utilized in the past were albumen, dextrin, casein or one of the soluble gums, all or which were serviceable, but incapable of producing the sharp images contemplated by the present invention. The inadequacy of such colloids to serve their assigned purpose was made more apparent when the copending application oi Vernon L. Gregory disclosed the addition of a fatty emulsion to the coat-ing composition. When fat was added to the composition in sufficient quantities to render the light sensitive colloid more receptive to ink, the coating image would be retained in an undesirabie relatively soft state and thereby incapable of readily producing the highest quality reproductions. Furthermore, such an amount of fat relative to the light sensitive colloid is apt to leave the entire surface of the lithographic plate in a somewhat greasy ondition with the attendant danger of ink being received by the unexposed plate portions which should be devoid of any ink. Also, as was pre viously pointed out, with some of the compounds previously used in the art, it was diificult and time consuming to effect the dissolution of the dried composition in Water.

It will be understood from the foregoin that three major problems ar presented in order to cure the above mentioned undesirable results. First, the coating composition must be easily soluble in water. Second, whether or not a fat is used in the composition, the coating image must be sufficiently hard to be capable of large numbers of reproductions. Third, if a fat is used in order to render the coating image more receptive to ink, some means must be provided to prevent the entire plate surface from becoming greasy.

We have overcome the foregoing and other problems by utilizing a water soluble caseinate in the colloid solution, the most readily available of which includes sodium caseinate, ammonium caseinate and potassium caseinate, but it will now be obvious to those skilled in the art that other soluble compounds of a metal with casein may be successfully substituted for the above examples. By using the above as the colloid base of the coating composition, it is apparent that the first problem of water solubility of the dried mixture is met. The other problems set forth are similarly met as the caseinate will act as a diluent in dispersing the fat concentration and will so react as to produce a hard, firm coating image.

As an example of a mixture which has been successfully tested, the followin is set forth in order to disclose to those skilled in the art the proper proportions of the various components thereof.

Example No. 1

300 parts by weight of a water soluble caseinate such as sodium caseinate, ammonium caseinate, or potassium caseinate, together with 350 parts by weight of ammonium bichromate or the potassium or sodium salts of bichromate which serves to render the caseinate light sensitive. To this mixture may be added 270 parts by weight of a fatty substance which will form an emulsion in water, such as dried milk, dried whole egg or natural soya bean powder. (If desired, the emulsion may comprise liquid whole milk or synthetic mixtures of a fat and an emulsifier.) If the foregoing weight parts are measured in ounces, suiiicient water is then added to make 680 quarts of solution.

The foregoing example will give excellent results, and it should be pointed out that while the use of sodium caseinate or similar soluble caseinat is most desirable when the composition includes a fatty substance, it may also be used without the addition of the latter. For example, if desired, the composition may merely include a soluble caseinate rendered light sensitive by the presence of ammonium bichromate or a potassium or sodium salt.

We have listed below certain percentage ranges for the ingredients which will produce a satisfactory composition, but it is believed that optimum results will be obtained when the caseinate represents approximately 60% of the total dry weight, the bichromate or other light sensitive material represents approximat 25%, and the dried fatty substance represents about 15% of the total weight. Regarding the latter perccntage, it is pointed out that the materials best suited for the fat, those forming a natural emulsion, such as dried milk have only about a 30% fat content, and, therefore, the actual fat content of the dry mixture is approximately 5% of the total weight.

Percentage ranges Caseinate to 80 Bichromate 5 to Fat 0.5 to 20 Further variations are possible, but it is believed that by providing the preferred amounts, a more satisfactory coating image will result.

We claim:

1. An image forming coating composition for photolithographic plates including a, caseinate selected from the group consisting of sodium caseinate, ammonium caseinate and potassium caseinate, a natural saturated fat; and a bichromate sensitizer.

2. An image formin coating composition for photolithographic plates comprising a water soluble caseinate, a light sensitive bichromate, and a natural saturated fat which will form a fatty emulsion in the presence of water.

3. An image forming coating composition for photolithographic plates comprising a dry mixture containing approximately by weight of a water soluble caseinate, approximately 25% by weight of a light sensitive material selected from the group consisting of ammonium bichromate and the potassium and sodium salts of bichromate, and approximately 5% by weight of a natural saturated fat selected from the group consisting of dried milk, dried whole egg, and natural soya bean powder.

4. An image forming coating composition for photolithographic plates comprising a water soluble caseinate; a bichromate light sensitizer; and a natural saturated fat capable of forming an emulsion in water; the said caseinat constituting from l0% to of the total dry weight, the said sensitizer constituting from 5% to 50% of the total dry weight, and the said fat constituting from 0.5% to 20% of the total dry weight.

References Cited in the file of this patent UNITED STATES PATENTS OTHER REFERENCES Clerc Technique Des Procedes Photomecaniques (19 17), Tome 1, published by Etablissements Bouzard-Calmels Paris, pg. 359. 

2. AN IMAGE FORMING COATING COMPOSITION FOR PHOTOLITHOGRAPHIC PLATES COMPRISING A WATER SOLUBLE CASEINATE, A LIGHT SENSITIVE BICHROMATE, AND A NATURAL SATURATED FAT WHICH WILL FORM A FATTY EMULSION IN THE PRESENCE OF WATER. 